Unaudited Quarterly Results of Operations - Schedule of Quarterly Results of Operations (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2018		Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Net Sales	$ 469,151		$ 228,483		$ 237,323		$ 240,385		$ 223,322		$ 214,623		$ 223,357		$ 215,435		$ 1,175,342	$ 876,737	$ 708,906
Gross Profit	151,690		71,940		78,255		74,226		68,470		69,013		72,126		66,167		376,111	275,776	222,132
Net income attributable to Altra Industrial Motion Corp.	$ (4,980)	[1]	$ 12,313	[1]	$ 19,007	[1]	$ 9,001	[1]	$ 12,440	[1]	$ 13,277	[1]	$ 15,384	[1]	$ 10,326	[1]	$ 35,341	$ 51,427	$ 25,140
Basic net income attributable to Altra Industrial Motion Corp.	$ (0.08)		$ 0.42		$ 0.66		$ 0.31		$ 0.43		$ 0.46		$ 0.53		$ 0.36		$ 0.93	$ 1.78	$ 0.97
Diluted net income attributable to Altra Industrial Motion Corp.	$ (0.08)		$ 0.42		$ 0.66		$ 0.31		$ 0.43		$ 0.46		$ 0.53		$ 0.36		$ 0.92	$ 1.77	$ 0.97
Restructuring Costs	$ 2,330		$ 610		$ 566		$ 943		$ 367		$ 680		$ 1,198		$ 1,898

[1] Includes restructuring costs by quarter